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Sealed Lead Acid Batteries


Sealed Lead Acid batteries come in a variety of technologies. Each technology has its attributes, advantages and disadvantages in any given application - however, they all remain 'Lead Acid' batteries.

They are known as;

  • SLA (Sealed Lead Acid) 
  • VRLA (Valve Regulated Lead Acid) 
  • AGM (Absorbed Glass Mat) 
  • Gel (Gelled electrolyte) 
  • Hybrid (Gel/AGM Hybrid)


Where a regular flooded battery has the electrolyte in a 'wet' fluid state, also referred to as 'free electrolyte', the essence of a sealed battery is to immobilise the electrolyte. However, it is necessary that the gasses generated during charging are recombined in a so called 'oxygen cycle' to the fluid and negative plate. Should the gasses escape, a gradual drying out would occur. To assist in the recombination of the water's constituents, oxygen and hydrogen, the sealed case is held in a positive pressure of 2 or 3lbs above atmospheric pressure.
In the case of rapid generation of oxygen gas exceeding the absorbing capacity, the pressure relief valve will open to release the excessive gasses.


SLA (Sealed Lead Acid) batteries otherwise known as VRLA (Valve Regulated Lead Acid) were originally known as "Dry Batteries". They were introduced in the 1950's and at that time untilised a Gel electrolyte. The otherwise free acid was immobilised with a fine silica powder and formed a gel substance. In the 1970's technology moved to AGM (Absorbed Glass Mat) where the separators between the plates were made of a felt of micro-fine glass fibers absorbing and immobilising the acid.


AGM has since become the preferred VRLA technology for use in stand-by or float applications. They are used in multi unit power packs for UPS support in the telecommunications, power, and indeed any mission critical industry where the power supply must not be interrupted.
Gel batteries are also used in UPS stand-by duty but it is argued that neither are the complete answer.
Hence the development of the Gel/AGM Hybrid

Gel technology has experienced good results in cyclic applications and used extensively in boats, motor homes, golf trundlers and carts - the list is long.

Nothing is perfect

The various shortfalls of AGM and Gel batteries are well documented.
Research has shown that considerable compressive force is needed for the AGM mat to make good electrolyte/plate contact. It looses compression as the cells age affecting performance.

On the other hand, the immobilised gel electrolyte suffers shrinkage over time due to loss of water and dry out or sulphating of the negative electrode. The gelled electrolyte inherently has a lower recombination efficiency compared with AGM.
And further, the gel will not tolerate gassing bought on by excessive voltage while charging. However, this issue is manageable with appropriate regulation of alternators and mains-powered chargers.

In spite of the problems discussed above, the AGM is still the preferred material for immobilising the electrolyte in VRLA batteries. To further improve gel technology, a colloidal poly-silica gel has been developed and this in combination with AGM technology has given rise to Hybrid Gel technology.

Excess gel surrounding the cell element provides additional reserve and superior thermal properties. And while the AGM-based technology has the preferred approach to immobilising the electrolyte, the combining of the two technologies is suggested as being the method to improve the sealed lead acid battery. Further development has been done to find a glass matting with better memory under compressive loads ensuring retention of plate contact for efficient ion transfer.

It has also been suggested that the Hybrid offers superior cyclic service even in adverse environmental conditions. Improved thermal properties extending long-term reliability and performance. However, as I have also suggested, nothing is perfect and it seems to include the Hybrid technology. Several installations have reported short life due to sensitivity of charging voltages and one manufacturer has ceased production altogether.

This takes us back to AGM technology and since it is now available for cyclic applications, I would suggest the Powersonic Deep Cycle range is as good as it gets until we find yet another technology from the world of electrochemistry.

Sealed batteries are arguably the future in Lead Acid technology and the technicians at BatteryDirect are leading specialists in Sealed Lead Acid electrochemical theory and applications.

View Sealed Lead Acid Battery Models and Pricing


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